In the present state of the art, point matrix-type cells using liquid crystals are generally controlled sequentially (multiplexing). Now, a sequential control mode of this kind can be applied only to cells making use of a threshold-type electro-optical phenomenon.
It is known that display cells comprising a liquid crystal, of the twisted nematic type, such as those described in U.S. Pat. No. 3.918.796, use a threshold-type electro-optical phenomenon. On the other hand, cells using a liquid crystal containing a dichroic compound, such as those described in U.S. Pat. Nos. 3.551.026 or 3.833.287, make use of an electro-optical phenomenon which does not have a clearly marked threshold. The result of this is that such cells, for control thereof, require a large number of output terminals, which makes their cost price prohibitive and which results in an excessive increase in their dimensions.
Further, even when it can be used, the sequential mode of controlling a liquid crystal display cell causes deterioration in the degree of contrast of the display, and also a reduction in the reading angle of the cell.
In order to overcome the above-indicated disadvantages, a display device has already been proposed (British Pat. No. 2.001.794), which comprises a point matrix cell and a control device including a circuit which is operative to produce n+1 different periodic signals, wherein n is the number of electrodes of a first system of the display cell. The signals must be such that the effective value (i.e. the root mean square) of the potential difference between any two of said signals is greater than or equal to the minimum electrical voltage required to produce an electro-optical effect in the sensitive material of the cell.
Each of the n first signals is applied to a respective electrode of the first system, and a signal selected from the n+1 signals described hereinbefore is applied to each of the electrodes of the second system which are associated with the columns of the matrix, the selection from n+1 signals being made in dependence on the information which is to be displayed by the cell.
Nonetheless, in this known display device, the matrix-type display cell is provided with electrodes which are structured in such a way that each column of points of the matrix is associated with a single electrode of the second system. The result of this is that the known cell can display only figures that can be represented by differentiation of a single point at most in each column of the cell. In consequence, the known display device cannot display symbols requiring differentiation of a number of points in each column, and in particularly cannot display alpha-numeric characters.
Also known (SID 1978 DIGEST, pages 92 and 93, an article entitled "Liquid crystal television display") is a point matrix-type display cell having a configuration of electrodes such that each column of points of the display matrix is associated with two electrodes of the second system. Nonetheless, this particular electrode configuration was proposed only for the purpose of reducing the degree of multiplexing in controlling the display cell.
The aim of the present invention is in particular to propose a point matrix-type display device capable of displaying figures such as alpha-numeric characters, which require differentiation of more than one point per column in the matrix, while making it possible to use a display cell which employs any electro-optical phenomenon.
In particular, an object of the present invention is to propose a point matrix-type display device which uses a display cell, the sensitive material of which is a liquid crystal comprising a dichroic compound.
Another object of the present invention is to provide a display device which gives an excellent level of contrast and a relatively large reading angle.